ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-10-1369-2010Ice nucleation and cloud microphysical properties in tropical tropopause layer cirrusJensenE. J.1PfisterL.1BuiT.-P.1LawsonP.2BaumgardnerD.31NASA Ames Research Center, Moffett Field, CA, USA2SPEC Inc., Boulder, CO, USA3Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Circuito Exterior, Mexico0502201010313691384This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/10/1369/2010/acp-10-1369-2010.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/1369/2010/acp-10-1369-2010.pdf

In past modeling studies, it has generally been assumed that the predominant mechanism
for nucleation of ice in the uppermost troposphere is homogeneous freezing of aqueous
aerosols. However, recent in situ and remote-sensing
measurements of the properties of cirrus clouds at very low temperatures
in the tropical tropopause layer (TTL) are broadly inconsistent with theoretial
predictions based on the homogeneous freezing assumption.
The nearly ubiquitous occurence of gravity waves in the TTL makes the predictions
from homogeneous nucleation theory particularly difficult to reconcile with measurements.
These measured properties include ice number concentrations, which are much
lower than theory predicts; ice crystal size distributions, which are much broader than
theory predicts; and cloud extinctions, which are much lower than theory predicts.
Although other explanations are possible, one way to limit ice concentrations is to
have on the order of 50 L<sup>&minus;1</sup> effective ice nuclei (IN) that could nucleate
ice at relatively low supersaturations.
We suggest that ammonium sulfate particles, which would be dry much of the time in the
cold TTL, are a potential IN candidate for TTL cirrus.
However, this mechanism remains to be fully quantified for the size distribution
of ammonium sulfate (possibly internally mixed with organics) actually
present in the upper troposphere.
Possible implications of the observed cloud microphysical properties for
ice sedimentation, dehydration, and cloud persistence are also discussed.